1. FIELD OF THE INVENTION
The present invention relates to a drive circuit for semiconductor light-emitting device. More specifically, this invention relates to a drive circuit for semiconductor light-emitting device which is applicable to a laser drive integrated circuit (IC) for high-speed switching used for, for example, magneto-optical disks and laser beam printers.
2. DESCRIPTION OF THE RELATED ART
FIG. 1 shows a laser drive circuit categorized as a cathode-common type circuit as known in the prior art. In FIG. 1, a component 1 is a base current source for determining the drive current to the laser semiconductor, which is ordinarily controlled so that the output voltage which is the quantity of laser light may be regulated to be a designated value by monitoring the quantity of laser light from the laser light-emitting device by means of a photo-diode not shown. Components 2 and 3 form a current mirror circuit composed of a couple of P-channel MOS transistors, where both of the drain and source terminals of the PMOS transistor 2 are connected to the base current source 1. The value of the drain current to the PMOS transistor 2 is reflected (mirrored) to the value of the output current from the PMOS transistor 3.
The drain terminal of the PMOS transistor 3 is connected to the terminal shorted both to the collector and base of the NPN transistor 4. The base of the NPN transistor 5 is connected to the terminal shorted both to the collector and base of the NPN transistor 4. With these NPN transistors 4 and 5, a current mirror circuit is formed. By making the ratio of the area of the emitter of the NPN transistor 4 to the area of the emitter of the NPN transistor 5 is 1:N, the value of the output current from the emitter terminal common to the transistors 4 and 5 may be obtained so as to be (1+N) times as large as the value of the drain current to the PMOS transistor 3.
A component 7 is a laser diode, and its cathode is connected to the ground level point (GND) 9, while its anode is connected the common-emitter terminal of the NPN transistors 4 and 5. The N-Channel MOS transistor 6 is a switching transistor which is turned on when the high-level signal is applied to the control signal input terminal, and to which the current supplied from the PMOS transistor 3 is led. In this state, a current is not supplied to the short-circuit terminal to the collector and base of the NPN transistor 4, and thus the current mirror circuit formed by the NPN transistors 4 and 5 is turned off. Therefore, the drive current for the laser diode 7 is zero. In addition, as the NMOS transistor 6 is turned off when the low-level signal is applied to the input terminal 10, the output current from the PMOS transistor 3 drives the laser diode 7 by driving the current mirror circuit composed of the NPN transistors 4 and 5. And then, the light emission from the laser diode 7 is switched in a high speed by turning the NMOS transistor 6 on and off in a high speed.
FIG. 2 shows another example of the semiconductor laser drive circuit as known in the prior art. In FIG. 2, the cathode of the laser diode 30 is connected to the lower electric potential terminal 22 in the circuit, and its anode is connected to the current mirror circuit 24. The input terminal to the current mirror circuit 24 is connected to the output terminal from the constant-current circuit 23. The emitter of the NPN transistor 25 which is switched by the control signal supplied from outside the circuit to terminal 32 is connected to the lower electric potential terminal 22, and its base is connected to the control signal input terminal 32 and its collector is connected to the output terminal from the constant-current circuit 23.
In the circuit shown in FIG. 2, the semiconductor laser diode 30 emits laser light in responsive to the current supplied to itself. Its light emission process is stated as below. At first, when the voltage applied to the control signal input terminal 32 is VH, the NPN transistor 25 is turned on and the current supplied by the constant-current source 23 is led to the NPN transistor 25 and is not supplied to the current mirror circuit 24 and hence as the current is not also supplied to the laser diode 30, and the laser light is not emitted.
Next, when the voltage applied to the control signal input 32 is VL, the NPN transistor 25 is turned off and the current supplied by the constant-current source 23 is led to the current mirror circuit 24 and hence, as the current is also supplied to the laser diode 30, and the laser light is emitted from the laser diode 30. At this time, the electric potential at the base of the current mirror circuit 24 is the sum of the voltage VF defined between the anode and the cathode of the laser diode 30 and the voltage VBE(ON) defined between the emitter and the base of the current mirror circuit 24.
However, in the prior art shown in FIG. 2, as the charge current for charging the junction capacitance in the laser diode, the following disadvantages are found.
(1) When the laser diode is turned on after a long period of time while which the laser diode is turned off, spike noises occur at the rise-up part of the waveform of the current supplied to the anode terminal of the laser diode. PA1 (2) The turn-on speed of the current when the laser diode is turned on after a longer period of time during which the laser diode is turned off is slower than the turn-on speed of the current when the laser diode is turned on after a shorter period of time during which the laser diode is turned off. PA1 input means for inputting a control signal; PA1 constant driving current generating means for generating the constant driving current; PA1 switching means for switching a supply of the constant driving current to the driving object in accordance with the control signal inputted by the input means; and PA1 means for supplying a current to the switching means in response to an input of the control signal representing a turn-on before the driving object is actually driven. PA1 an inverter may be provided between input means and the supplying means, or between the input means and the switching means. PA1 means for generating a drive current; PA1 means for switching an output of the drive current to an output terminal; and PA1 means for generating a bias current, PA1 in which the bias current generated by the bias current generating means is supplied to the output terminal independent at a state of the switching means.
The factors leading to the above disadvantages (1) and (2) are described below.
FIG. 3 shows an equivalent circuit of the laser diode 7. In FIG. 3, A is the anode terminal, K is the cathode terminal, D is the junction part of the laser diode, and C is the junction capacitance of the laser diode. In case that the laser drive circuit shown in FIG. 1 drives the laser diode 7, the current is led to the laser diode junction part D when the voltage VF defined between the terminals of the laser diode rises to about 1 to 1.5 V. That is, the drive current in the laser drive circuit is at first used for charging the junction capacitance C, and when VF rises up to the voltage at which the laser light can be emitted from the laser diode, the fraction of the current used for charging the junction capacitance C is reduced and the effective current is led to the junction part D of the laser diode.
FIGS. 4A-4C show waveforms of currents supplied into the laser diode shown in FIGS. 1 and 2. The waveform (FIG. 4A) in FIG. 4 refers to the current Icj led to the junction capacitance C, the waveform (FIG. 4B) refers to the current Ild led to the laser diode junction part D, and the waveform (FIG. 4C) refers to the current ILD led to the anode terminal A. In the time domain, the current ILD led to the anode terminal A is the sum of Icj and Ild, and hence, the peak of the waveform of the current Icj used for charging the junction part D makes a noise on the current ILD led to the anode terminal A.
Next, the factor leading to the above disadvantage (2) is described below.
FIGS. 5A-5D show waveforms of currents and voltage with the turn-off time period of the laser diode shown in FIGS. 1 and 2 changed. In this figure, the waveform (FIG. 5A) refers to the laser drive current ILD led to the anode terminal A of the laser diode 7, the waveform (FIG. 5B) refers to the voltage VF, the waveform (FIG. 5C) refers to the charge current Icj for charging the laser diode junction capacitance C, and the waveform (FIG. 5D) refers to the current Ild led to the junction part D of the laser diode.
Individual waveforms show differences in the time-domain behavior in responsive to the change of the turn-off time period of the drive current of the laser diode.
The pulse P1 in the waveform in FIG. 5A is a step response of the laser drive current when the laser diode is driven after a long time Toff1 during which the laser diode has been turned off. That is, Toff1 is far greater than Toff2. As shown in the waveform in FIG. 5B, while the time period Toff1 during which the laser diode is turned off in a long time, the electric charge stored in the laser diode junction capacitance C is almost discharged with a small quantity of Off current, and hence, VF goes down to the GND level. Therefore, the additional current is required for charging the laser diode junction capacitance C when the laser diode is turned on next, and the charge current flows in the waveform in FIG. 5C. Eventually, the current led to the anode terminal A takes a step response shaped in the pulse P1 containing a noise in its rise-up part.
As for the pulse P2 formed after a short period Toff2 after the fall-down of the pulse P1, as the laser diode is turned on before the electric charge stored in the laser diode junction capacitance C is fully discharged, the voltage VF does not fall down to the GND level at the beginning of the pulse P2 as shown in the waveform in FIG. 5B, and the charge current shown by FIG. 5C flows a little. Therefore, the current ILD led to the anode terminal A does not contain noises. In addition, as the pulse P2 does not need the current Icj, the turn-on speed of the pulse P2 is faster than the pulse P1.
In summing up, in the laser drive circuit in the prior art shown in FIG. 1, after a long period of time during which the laser diode is turned off, a spike noise is contained in the laser current waveform in its rise time part, and its turn-on speed is restricted.
These disadvantages described above are found in another prior art shown in FIG. 2. Parasitic capacitors are formed at the output terminal of the constant-current circuit 23, the collector of the switching NPN transistor 25, and the collector and base of the current mirror circuit 24, and individual parasitic capacitors are charged by the output current from the constant-current circuit 23 when the switching NPN transistor 25 is transferred from the turn-off state to the turn-on state. And then, a designated quantity of current is led to the current mirror circuit 24 when the electric potential at the base of the current mirror circuit 24 rises up from VCE(SAT) to VF+VBE(ON), and finally, the laser diode 30 emits laser light with its intensity determined in responsive to the current led to the laser diode.
Due to the time constant developed in the time domain behavior for charging the above described capacitances, the laser diode 30 emits laser light with a delayed time after the voltage applied to the control input terminal 32 changes from VH to VL. Especially, in case of reducing the output current from the constant-current circuit 23 in order to reduce the output power of laser light, the delayed time for emitting laser light is remarkable and can not be ignored for attaining the high-speed switching response of the laser light.